Golf ball designers are interested in determining various aerodynamic and mechanical characteristics for golf balls. One such characteristic indicative of golf ball performance is the coefficient of restitution (CoR) from impacting golf clubs. The coefficient of restitution is the ratio of the relative velocity between two objects after direct impact to the relative velocity before impact. As a result, the CoR can vary from 0 to 1, with 1 being equivalent to a perfectly or completely elastic collision and 0 being equivalent to a perfectly plastic or completely inelastic collision. Since a ball's CoR directly influences the ball's initial velocity after club collision and travel distance, manufacturers are interested in this characteristic for designing and testing golf balls.
One conventional technique for measuring CoR uses a golf ball or golf ball subassembly, air cannon, and a stationary vertical steel plate. The steel plate provides an impact surface weighing about 100 pounds or about 45 kilograms. A pair of ballistic light screens, which measure ball velocity, are spaced apart and located between the air cannon and the steel plate. The ball is fired from the air cannon toward the steel plate over a range of test velocities from 50 ft/s to 180 ft/sec. As the ball travels toward the steel plate, it activates each light screen so that the time at each light screen is measured. This provides an incoming time period proportional to the ball's incoming velocity. The ball impacts the steel plate and rebounds though the light screens, which again measure the time period required to transit between the light screens. This provides an outgoing transit time period proportional to the ball's outgoing velocity. The coefficient of restitution can be calculated by the ratio of the outgoing transit time period to the incoming transit time period. A drawback of this method is that the 45 kg steel plate is a poor simulation of a 200 gram golf club.
A CoR measuring method employed by the U.S.G.A. uses a golf ball or golf ball subassembly, a launching device, and a substantially fixed titanium disk. The titanium disk intending to simulate a golf club is circular, and has a diameter of about 4 inches, and has a mass of about 200 grams. The disk is mounted on an X-Y-Z table so that its position can be adjusted relative to the launching device prior to testing. A pair of ballistic light screens are spaced apart and located between the launching device and the titanium disk. The ball is fired from the launching device toward the titanium disk at a predetermined test velocity. As the ball travels toward the titanium disk, it activates each light screen so that the time period to transit between the light screens is measured. This provides an incoming transit time period proportional to the ball's incoming velocity. The ball impacts the titanium disk, and rebounds through the light screens which measure the time period to transit between the light screens. This provides an outgoing transit time period proportional to the ball's outgoing velocity. The CoR can be calculated using the mass of the ball, the mass of the disk, outgoing time difference and incoming time difference.
The U.S.G.A. method also has drawbacks. In order to obtain useful data, the ball must impact the titanium disk at its center. Due to the size of the disk and the inaccuracy of the air cannon, the location of the titanium disk on the X-Y-Z table must be adjusted by trial and error until the ball impacts the plate at the center. Since the air cannon is inconsistent, many shots miss the center of the disk and are rejected. Furthermore, examination of the disk and ball impact position is time consuming, and numerous shots are required to obtain a statistically significant population of central impacts to determine the CoR. This required set-up is time consuming and undesirable.
Therefore, a need exists for a method and apparatus for measuring the CoR of a golf ball after a collision with a simulated club that reflect the actual mechanics of the real-world golf swing, as closely as possible. This method also preferably minimizes potential problems with aiming the ball at a relatively small-simulated club.